Gap junctions and Aquaporin4 (AQP4) water channels form a water distribution network that is crucial for brain water homeostasis and for generation of water movements hypothesized to drive the glymphatic circulation. Despite their importance, fundamental issues remain to be resolved regarding the nature of the channels formed by AQP4 and its newly discovered isoforms and the interdependence of gap junction and AQP4 distribution in astrocytes. We here propose to determine function and structure of novel AQP4 isoforms and interactions with gap junction plaques using transfected cells and transgenic mouse models that were newly generated by the MPIs. One particular focus will be on the AQP4 isoforms that aggregate into Orthologal Arrays of Particles (OAPs) in astrocyte endfeet and are believed to provide most water flux. Because AQP4 channels and gap junctions are primarily in the endfeet of astrocytes, they provide control of brain water homeostasis. We will test the hypothesis that changes in AQP4 OAPs correlate with changes in gap junction plaque structure/distribution and that the permeability of blood-brain-barrier (BBB) is modified by this interplay between the astrocyte AQP4 isoforms and gap junction plaques. We expect that these straightforward studies using methods routine in our laboratories on new mouse models will provide key fundamental information on the organization of AQP4 isoforms in astrocyte endfeet relative to gap junction plaques and on the functions of AQP4 isoforms and gap junctions in BBB regulation. This proposal represents a collaboration between groups of investigators who have published several studies together and is expected to provide the foundation for future studies aimed to understand the cellular mechanisms involved in the redistribution of water, ions and solutes mediated by the combined action of AQP4 and gap junctions. .